Gas separation membranes employing a thin polymeric film have been extensively studied for a wide array of applications. For many gas separation membranes, a thin film is applied to a flat porous substrate, wherein the film contributes the permselective properties to the combination.
The thin film effectuating the gas separation may also be applied in the form of a coating on a porous substrate, such as microporous hollow fibers, which in a bundle is commonly referred to as a hollow fiber module. The microporous hollow fiber substrate may be organic, inorganic, or organo-metallic.
Various polymers have been used as the thin film for gas separations, though researchers are yet to discover a thin film forming polymer that achieves both good gas selectivity and good permeability in order to meet pressing industrial demands. Moreover, conventional polymers offer limited options in the development of suitable polymer-based gas separation films due to the complexity of synthesis, lack of film formation characteristics, poor solubility or chemical resistance, and cumbersome application techniques.
Among conventional polymers, polyorganosilicones, in general, have been targeted for certain applications due to their biocompatibility, low coefficient of friction, and ease of production. Depositing an ultra-thin film of conventional polyorganosilicones, however, remains a challenge. As a consequence, membranes synthesized from conventional polyorganosilicones exhibit low gas permeability (due to their relatively high thickness), poor gas selectivity, poor mechanical properties and poor adhesion.
Plasma polymerization of organic compounds has been used, as an alternative technique, to obtain thin film coatings that are free from pollutants or unwanted byproducts. Most plasma-derived polymers have good chemical resistance, and exhibit strong adhesion to the underlying substrate. The ability to deposit a film with extremely low thickness using this technique also lends advantages in the construction of gas separation membranes of high gas permeability.
Plasma polymerized polymer coatings therefore overcome some of the drawbacks of conventional coating techniques, as they can be deposited as an ultrathin film and provide good gas permeability but most plasma polymers suffer from a low rate of polymer deposition, insufficient pore coverage and hence inferior gas separation characteristics. Most plasma polymers also suffer from poor shelf life in the form of degraded permeability characteristics as they continue to interact with the atmospheric oxygen. Thus, organosilicone-based coatings of high gas selectivity and high gas flux have remained elusive, and have therefore not been widely used in applications in industry where high flux-selective gas permeation is sought.
Among the organosilicone compounds, plasma polymerized organosiloxanes have received particular attention of plasma researchers due to their structural similarity to conventional silicone rubber. Commercially useful coatings have been prepared from plasma polymerization of tetramethyldisiloxane (TMDSO) and hexamthyldisiloxane (HMDSO) monomers for applications in the biomedical areas, for example, for providing lubricity to the substrate. Both TMDSO and HMDSO are relatively low molecular weight compounds that can be easily volatized in the plasma chamber, which is why they have been widely used for plasma polymerization. These monomers, however, suffer from a low rate of polymer deposition, and thus poor substrate pore coverage, and therefore find limited use in gas separation applications.
The use of high molecular weight Organosiloxanes, with boiling points in excess of 100 degree C., has generally been avoided due to the fear of low vapor pressure and hence even lower rate of polymer deposition. U.S. Pat. No. 5,463,010 to Hu et al. describes using hydrocyclosiloxane monomers such as 1,3,5,7-tetramethylcyclotetrasiloxane (B.P. 134-135° C.) for polymeric coatings to a substrate. While such monomers have increased molecular weight, membranes constructed from such monomers are found to exhibit poor aging characteristics. In some applications, fluorinated organic compounds have been co-polymerized with the organosiloxanes to improve hydrophobicity, abrasion resistance and polymer deposition rate. Such fluorinated copolymers, however, suffer from poor bondability to common substrates and adhesives, particularly in membrane applications. As a result, leakage is a commonly-cited problem in membrane modules fabricated from fluorinated copolymers of low molecular weight organosiloxanes.